The present invention relates to an apparatus and a method of fabricating a specimen for observation of a structure, which is used to observe a cross sectional shape of a device or the like.
There have been increased needs for examination and analysis of semiconductor devices, for which miniaturization is going to progress. In a failure analysis, which specifies the cause of failure, among the needs, it is an essential technology to directly observe a defect inside a device. Method of observing a section is shown in FIGS. 2A and 2B. In the beginning, focused ion beam (referred below to as FIB) 201 is used to process a rectangular hole 202, of which one side defines a position being requested for section observation. One side of the hole formed thereby defines a requested section 203, and the section is observed by a scanning electron microscope (referred below to as SEM). Since the section is formed by irradiating FIB 201 in parallel to the requested section, the section formed by FIB is essentially flat. On the other hand, SEM used for observation irradiates primary electrons in a position of observation and imaging, as contrast, the number of secondary electrons (or reflected electrons) generated from the position to thereby form an image to be observed. While the number of secondary electrons depends upon material of an object, it depends upon an irregular shape further than that. That is, since an observation section 203 formed by FIB is flat as described above and little irregular in shape, a difference in SEM image contrast consists of only a difference in secondary-electron yield attributable to a material. However, only a difference, in secondary-electron yield, attributable to a material is insufficient for that observation of a minute structure, the necessity of which has been increased in recent years, and so there is caused a problem that the image resolution is insufficient.
Therefore, it is desirable to emphasize a contrast difference attributable to a structure. As measures for realizing this, there is used decoration of profile by forming irregularities every material of a structure. In SEM, when irregularities are present, boundaries of raised portions are observed to be bright since edges become large in secondary-electron yield, so that observation in high contrast is made possible. In order to provide for a difference in level every structure, a difference in processing sputtering yield, attributable to a material, is made use of. In case of FIB, since a difference in sputtering yield, attributable to a material, is present also in physical sputtering, irregularities 301, 302, 303 every material can be formed as shown in FIGS. 3A and 3B by irradiating FIB not in parallel to a section but obliquely at a certain angle to a section. FIG. 3B shows a section when a section indicated by a broken line in FIG. 3A is viewed along an arrow 304, and a left side thereof defines an outermost surface. For example, structures 301, 303 are made of Si and a structure 302 is made of SiO2. Here, the reason why FIB is irradiated obliquely is that since a section is defined by a side of a processed hole, irradiation perpendicular to the section is impossible. Since a difference in sputtering yield, attributable to a material, is relatively small, however, an intense processing on a section is needed to form sufficient irregularities by physical sputtering with FIB, which causes a problem in terms of damage.
Therefore, methods of efficiently forming irregularities depending upon a material include a processing making use of chemical reaction. For example, Japanese Patent No. 3216881 indicates that irregularities due to differences in sputtering rate can be formed by performing a FIB processing while making fluorine-containing gases flow to a processed sample. Japanese Patent No. 3350374 discloses FIB assist-etchant measures, in which halogenated gases and halogenated hydrocarbon gases such as Cl2, XeF2, CF4, CHF4, C2F6, C3F8, C4F8, etc. are used as etch-assisting gases. U.S. Pat. No. 6,211,527 discloses measures to realize decoration of a section by means of FIB assist-etchant, in which halogenated hydrocarbon gases are used as etch-assisting gases.
By performing FIB assist-etchant with the use of etch-assisting gases as in the related art, it is possible to vary a processing speed every material. As described above, a difference in level of irregularities is important in order to provide for a difference in contrast in SEM observation. However, an excessive processing with a view to forming a difference in contrast is not desirable. The reason for this is a fear that in case of FIB, since the presence of irregularities brings about an increase in sputtering rate at edge portions, the edge portions get out of shape to look differently in SEM observation from an original structure of a section. Further, digging much by means of FIB in order to provide for a difference in level means that a structural profile different from a section being essentially observed is observed in a device, which is varied in structure in a depthwise direction. Therefore, it is desirable to make an amount of processing as small as possible to form a necessary difference in level.
Gases adopted in Japanese Patent Nos. 3216881 and 3350374 are in essence gaseous in a standard state. The standard state means 1 barometric pressure and 25° C. Essentially, these gases are those used in plasma etchant. In case of plasma etchant, gases themselves are ionized as plasma and ionized molecules are accelerated by plasma sheath formed on a surface of a sample, being a target of processing, to be irradiated on a surface of the sample, thereby performing etchant. However, FIB assist-etchant is different in reaction configuration in the following manner. Etch-assisting gases as supplied are first adsorbed by a sample surface. FIB is irradiated on the sample surface to inject energy thereinto to give thereto a reaction energy for reaction of a constituent material of a sample with gases, thereby generating a chemical reaction to subject a material of the sample to etchant. Therefore, in order to adequately cause the reaction, it is required that gases be sufficiently adsorbed by the sample surface. However, the probability that the gases being originally gaseous at room temperature are physically adsorbed by the sample surface is small as compared with a substance being originally solid, so that it is difficult to ensure a sufficient adsorbed amount. That is, gases used in plasma etchant is not necessarily suited to FIB etch-assisting gases.
Further, a material accounting for a large part of a semiconductor device comprises silicon (Si) being a substrate material and an oxide silicon (SiO2, etc.) being an insulating material. While it is desirable to form a difference in level between Si and SiO2, the reason why the CF gases are used is as follows. Si reacts chemically with F to generate volatile SiF4 or the like to be etched. In this case, however, C remains whereby a substance, such as SiC, etc., being hard to be etched is formed to suppress etchant. On the other hand, with SiO2, Si volatilizes as SiF4 or the like as described above and C also reacts with O to volatilize as CO2 or the like, so that etchant is not suppressed like Si. Thereby, Si and SiO2 are varied in sputtering rate, so that it becomes possible to create a difference in level. CF gases, such as CO, COOH, etc., which contain O, are adopted in the U.S. Pat. No. 6,211,527. While SiC serves to suppress Si etchant in the reaction, O contained in etchant gases involves a fear that such suppressing effect is decreased. Therefore, there are needed an apparatus and a method of fabricating a section, in which the problems are solved, an effective difference in etchant is ensured, and a requested difference in level is fabricated in less processing to enable realizing a high contrast observation with SEM.